No abstract

Whole-cell patch-clamp recordings were obtained from light-responsive on-bipolar cells in retinal slices of the dogfish. Inclusion of the G-protein activator, GTP gamma S, in the intracellular patch solution mimicked the action of glutamate, inducing an increase in net outward current (interpreted as a decrease in inward current), a decrease in membrane conductance and block of light responses. Cyclic GMP (cGMP) in the patch pipette increased inward current and membrane conductance, and blocked light responses. Cyclic AMP had no effect. IBMX, a phosphodiesterase inhibitor, produced the same effect as cGMP, suggesting the presence of a cGMP phosphodiesterase in rod bipolar cells. These results indicate that the glutamate receptors of on-bipolar cells are coupled via a G-protein to regulate intracellular cGMP, which, in turn, results in the opening of sub-synaptic membrane channels. The similarity to phototransduction is striking, and the proposed scheme would account for the high gain in transmission of rod signals to on-bipolar cells.

No abstract

The rod visual system is extraordinary not only in its great sensitivity in the dark-adapted state, when it is capable of the detection of a few photons, but also in its ability to operate over a range of light intensities about a million times greater than absolute threshold (Aguilar & Stiles, 1954). High sensitivity in the dark-adapted state is achieved by high gain in phototransduction in rods (Pugh & Lamb, 1993) and in synaptic transduction in 'on' bipolar cells (Shiells & Falk, 1994), each involving a cGMP cascade linked to rhodopsin or to a metabotropic glutamate receptor (mGluR), respectively. This mGluR has been cloned and identified as mGluR6 (Nakajima et al. 1993). Psychophysical studies have indicated that the visual threshold rises at background light intensities too weak to induce significant adaptation in rod photoreceptors. Thresholds double at backgrounds when only one out of three rods absorb one photon per second, remarkable given that each rod in the human eye contains about 10Ì rhodopsin molecules (Rushton, 1965). To operate over such a wide range of light intensities there appears to be light-and time-dependent control of gain, or adaptation, in both photoreceptor and 'on' bipolar cell cGMP cascades. Rod photoreceptors are relatively depolarized in the dark, releasing glutamate from their synaptic terminals at a high rate. Glutamate activates the 'on' bipolar cell mGluR6, which in turn activates a G-protein and phosphodiesterase (PDE) leading to the hydrolysis of cGMP and thus a reduction in cGMP-activated conductance. The closure of cGMP-activated cation channels results in a relatively hyperpolarized state in the dark. Light hyperpolarizes rods,

Rod on-bipolar cell light responses are mediated by a class of metabotropic glutamate receptor which is coupled via a G-protein to the control of a cGMP cascade, with cGMP acting to open cation channels, whilst off-bipolar cells possess ionotropic glutamate receptors. Whole-cell voltage-clamp recordings were obtained from on-and off-bipolar cells of dark-adapted dogfish retinal slices, identified by their light responses. Isolated cells were exposed to concentration-jumps of glutamate. At negative voltage-clamp potentials, on-bipolar cells responded to glutamate with outward currents with a mean delay of 10.8 ms, whilst off-bipolar cells responded with inward currents without any delay. Neither cell type showed desensitization to applied steps of glutamate. The dose-response relation for on-bipolar cells showed no gradual saturation, but increased linearly with a sharp cutoff above 200 nM glutamate. This dose-response relation could be fitted with a theoretical expression assuming Michaelis-Menten kinetics for the action of glutamate on receptors and a linear relation between the concentration of receptors bound to glutamate and the fall in cGMP this induces. The dose-response relation of off-bipolar cells showed saturation with a limiting slope of 2 at low glutamate concentrations, suggesting that two molecules of glutamate are required to open each channel by a cooperative mechanism. The glutamate receptor coupled cGMP cascade of rod on-bipolar cells can account for high synaptic voltage gain.